Fran Halsall and James Goddard both negotiated the first steps towards fulfilling their Olympic dreams, while the women's 4x200 metres freestyle - minus Rebecca Adlington - qualified for tonight's final at the Aquatics Centre.
It also emerged that US President Barack Obama tweeted his personal congratulations to Michael Phelps after the American swimmer set a new Olympic record of 19 medals.
BarackObama tweeted: "Congrats to Michael Phelps for breaking the all-time Olympic medal record. You've made your country proud. -bo," to which the Baltimore swimmer replied: "Thank you Mr. President!! It's an honor representing the £USA !! The best country in the world!!"
Much was initially made by some of Adlington's absence this morning.
But given that she is just the fourth fastest Briton in 2012 in this event - and 51st in the world - the news of her omission, and any effect it may have had, has been exaggerated.
Where possible British Swimming will rest swimmers who have individual races to prepare for, and the Bill Furniss-coached star takes to the water for the 800m freestyle heats tomorrow morning.
Caitlin McClatchey, Rebecca Turner and Eleanor Faulkner have all gone quicker this year and the trio, along with Adlington's close friend Jo Jackson, made up the squad that qualified seventh overall in seven minutes 54.31 seconds.
The line-up for tonight's final can change and Adlington may replace one of the four, a decision that will be made this afternoon.
One reason for the reaction to Adlington's omission - apart from her high profile - may have been the memory of Beijing.
Adlington and McClatchey were both rested for the heats, replaced by medley specialist Hannah Miley and sprinter Fran Halsall.
However, that was not the case today with Jackson bringing the squad home as both she and Turner made up for disappointing swims in their individual races earlier in the week.
Jackson, who failed to make the 400m freestyle final four years after picking up the bronze medal, said: "It's been a tough week. It hasn't gone to plan. I was grateful they put me in the heat to give me another shot after the 400. The 400 didn't go to plan. I was absolutely gutted but I came out there for the team today and hopefully I did a good job."
Halsall was distinctly unimpressed with her performance in the 100m freestyle, her time of 54.02 seeing her through seventh overall.
The 22-year-old said recently she prized a gold in what she described as the blue riband event above anything else.
However, after today's heat, the Loughborough ITC athlete said: "I made the semi. That's all that matters. Other than that it felt s***.
"Let's see what happens. Maybe I will swim faster.
"It will be 53 to make it so it won't be that much, but hopefully I will be able to make the final.
"The crowd were good for me. They really pull you along. It was really enjoyable to have them there.
"I would like to dominate this event and I will try my best to do so but I can't control what anyone else does so I am just looking to giving my performance and hopefully tomorrow as well."
Team-mate Amy Smith also made it through in 13th in 54.37, with China's Yi Tang quickest.
For Goddard, London represents the last chance of an Olympic medal after finishing a heartbreaking fourth in the 200m backstroke in Athens and sixth in the 200m individual medley four years ago.
The Stockport ITC swimmer is concentrating solely on the medley given the proximity of the events in the schedule, especially in relation to an ongoing shoulder problem.
The event has been billed as a showdown between Phelps and fellow American Ryan Lochte, although Beijing silver medallist Laszlo Cseh will be looking to gatecrash the party.
Goddard was fourth behind the trio at last year's World Championships and second to the Hungarian in his heat in 1:58.56, sixth overall into tonight's semis.
He said: "I suppose I am pretty happy with that. It was my first swim for a while on day five so I was really pleased to get it out of the way. A bit of rest and I will be ready for tonight."
Of facing the two Americans, the 29-year-old added: "I have to be confident. If you aren't confident, you're not going to do anything so... I just have to stick to my game plan and if I get it right then... we will see what happens.
"It's a task, but no-one said it was going to be easy. Tough tasks are what athletes look for. We look for challenges we want to overcome and this is just one of them."
Joe Roebuck was ill only a few weeks ago but he also made it through in 15th in 2:00.04, and he said: "I am just really happy to be here, so to be coming back tonight, I can't wait to walk out there."
Marco Loughran and Chris Walker-Hebborn failed to make it through to the 200m backstroke semis, while Stacey Tadd was two places off qualification in 18th in the 200m breaststroke.